Sinamics S120 CU310-DP STARTER Communication PROFIBUS

Problem Statement

During initial commissioning of a SINAMICS S120 drive system built around a CU310-DP Control Unit, the engineering laptop running Drive ES STARTER V4.1.2.4 cannot establish an online connection to the drive. The Accessible Nodes scan returns no PROFIBUS participant even though the USB PC Adapter is active and configured at 1.5 Mbps, address 0. The drive itself is fully operational: the RDY LED on the CU310 is solid green after power-up, and parameter changes committed through the BOP20 operator panel are accepted and stored in the non-volatile memory of the Control Unit. The fault is therefore in the engineering link between STARTER and the CU310, not in the power section or the drive firmware.

This reference documents the diagnostic and corrective procedure for this class of fault, the most common root causes observed on S120 commissioning sites, and the firmware/component compatibility constraints that must be respected when Drive ES STARTER (a pre-TIA commissioning suite) is used on a CU310-DP.

Affected Hardware and Software

Confirm the on-site inventory against the table below before starting any diagnostic. Mismatches between the installed STARTER components, the PC adapter firmware, and the CU310 firmware are the single largest source of "no node found" faults.

Physical Interfaces on the CU310-DP

The most common commissioning error on a CU310-DP is to plug the PC adapter cable into the wrong socket. The CU310-DP exposes three service-relevant connectors that must be distinguished before any software configuration is started.

The PC Adapter USB (6ES7-972-0CB20-0XB0) must be connected to X121, the 9-pin D-sub PROFIBUS socket. Connecting it to the mini-DIN X124 will never produce a PROFIBUS link and STARTER will not be able to enumerate the node regardless of the address, baud rate, or "PG is the only master" setting. A null-modem RS232 cable into X124 is the correct wiring only when using the SINAMICS PC connection kit or a third-party serial commissioning tool talking raw USS/PPI frames at 19.2 kbps.

The X121 pinout is the standard PROFIBUS DP assignment:

Root Cause Matrix

The following table maps the most common causes observed on commissioning sites to the observable symptom. Walk this matrix top-to-bottom during diagnosis.

Pre-Flight Checks Before Touching Software

1. Confirm the RDY LED on the CU310 is solid green. If it is yellow, red, or off, the drive is not in RUN and the PROFIBUS stack is not active — fix the drive state first (complete basic commissioning via BOP20) before attempting to go online.
2. Confirm the cable is physically connected to X121 and not to X124 or any RJ45 socket.
3. Confirm the PC Adapter USB shows a steady green "Link" LED. The "Activity" LED should flicker on every poll.
4. Verify the terminating resistor is enabled in the PC Adapter (slide switch on the housing) and that the far end of the segment is also terminated. The CU310 itself does not provide a bus terminator; a separate PROFIBUS connector with integral termination is required at the last device.
5. Confirm the drive PROFIBUS address (parameter p0918) by reading it on the BOP20: navigate to p0918 and record the value. A typical default is 3, 4, 5, or 6 depending on the SSP. STARTER will only find the drive at the address that physically matches p0918.

PG/PC Interface Configuration

The PG/PC interface is a Windows-level setting that routes all DP traffic through a single device handle. The configuration must match the physical USB-to-PROFIBUS adapter exactly.

1. Open Control Panel → Set PG/PC Interface (in the Siemens installation: Start → SIMATIC → STEP 7 → Set PG/PC Interface or in pure Drive ES: Start → Drive ES → Set PG/PC Interface).
2. Under Access Point of the Application, select S7ONLINE (this is the default for STARTER commissioning; do not use CP_L2_1 unless your project requires it).
3. Under Interface Parameter Assignment Used, choose PC ADAPTER (PROFIBUS) — the entry that matches MLFB 6ES7-972-0CB20-0XB0.
4. Click Properties and set:
   • PG/PC is the only master on the bus: enabled. With a S7-300/400 controller and the PC attached to the same segment, the PG enters the bus as a Class-2 master; if the PLC is in stop or the bus master is not generating token frames, the PG must be promoted to "only master" to gain the token.
   • Transmission rate: 1.5 Mbps (matches the reported setting) or Auto for first scan.
   • Highest station address (HSA): 126 (matches the PROFIBUS standard limit).
   • PG/PC address: 0 (PG is master, address 0 is reserved for class-2 masters and is the safe default).
   • Timeout: 30 s (matches the reported setting; do not lower below 10 s for cold scans).
5. Click OK → OK to commit. Windows may re-bind the USB device — disconnect and reconnect the PC Adapter if the activity LED does not resume within 5 s.

STARTER Online Procedure

After the PG/PC interface is correctly set, drive the online connection from inside STARTER. The sequence below applies to Drive ES STARTER V4.1.2.4 and is equivalent in later Startdrive versions.

1. Launch STARTER. If a project for this drive already exists, open it. If not, create a new offline project and insert the CU310-DP with the correct order number (6SL3040-0LA00-0AA0 or -0LA01-0AA0 depending on hardware revision).
2. Right-click the drive in the project tree and choose Target device → Online → Accessible nodes. STARTER broadcasts an FDL status request at all valid addresses between 0 and the configured HSA. The scan typically takes 5–25 s.
3. If the drive responds, it appears with its PROFIBUS address and diagnostic text. Highlight it and click Go online. STARTER uploads the actual parameter set and switches to the online view.
4. If no node appears, switch to Target device → Online → Online via TCP/IP as a fallback path. The CU310-DP does not support this directly — use a CU310-PN variant (X100) or a CU310-2 DP (X125 Ethernet service port) for direct Ethernet access. The original report has neither, so this path will not help a CU310-DP without additional hardware.
5. When online, immediately read p0918 to verify the address and r2050[0…7] to verify the controller is sending PZD telegrams. If r2050 is all zeros, the drive is online but the PLC is not in RUN, or telegram selection is not configured.

Firmware and Software Compatibility

Drive ES STARTER V4.1.2.4 with internal identifier 47.52.11.00 supports the SINAMICS S120 firmware range that was current at the time of release. The minimum constraints are:

• CU310-DP firmware versions in the SINAMICS S120 V2.x / V4.x / V5.x families are supported, but each release line requires a corresponding SSP. With Drive ES STARTER V4.1.2.4, the latest S120 firmware that can be brought online is generally the V4.x line. V5.x and later SINAMICS firmware is supported by Startdrive in TIA Portal V13 SP1 / V14 / V15 onward — not by Drive ES STARTER V4.1.2.4.
• If the CU310 is shipping with firmware V5.2 or newer, the project will go online only with Startdrive (TIA Portal). Drive ES STARTER V4.1.2.4 will show the device as "incompatible" or will fail to read the parameter tree.
• DCC_SINAMICS V2.0.2.4 is the matching Drive Control Chart editor for S120 V4.x. It must be installed even if no DCC chart is in use, because STARTER loads DCC libraries at startup and will not enumerate the project tree without them.

Diagnostic Parameters via BOP20

When the bus link is dead, the BOP20 is the only local interface that can be used to interrogate the CU310. The most useful parameters are listed below. The discussion thread suggested reading r0963 — that parameter exists in the S120 parameter list but its meaning varies by firmware; the more universally useful diagnostics are listed here.

To navigate the BOP20 to a parameter, use the following key sequence:

1. Press the FN key until the display shows p0000 or the parameter number.
2. Use the up / down arrow keys to scroll to the target parameter (for example p0918).
3. Press P to read the value. To change it, press P again to enter edit mode (the colon appears), use the arrow keys to set the new value, and press P to confirm.

For setting p0918, a 24 V power cycle of the CU310 (or a complete restart via the BOP RUN → STOP → RUN sequence) is required before the new PROFIBUS address becomes active. STARTER will not see the new address until the bus stack re-initializes.

Hardware and Wiring Verification

1. Inspect the PROFIBUS cable. It must be a purple (violet) shielded twisted-pair cable with characteristic impedance 150 Ω. Telephone-grade cable will not work, even at 187.5 kbps.
2. Verify the segment length. At 1.5 Mbps, the maximum trunk length without repeaters is 200 m; at 12 Mbps it drops to 100 m. The PC Adapter cable is a 3 m drop line — this is acceptable only if the trunk length is below the limit.
3. Measure termination with the bus powered off: between pins 3 (B-line) and 8 (A-line) at each end of the segment, the resistance should read 220 Ω. Any other value indicates a missing or duplicated terminator.
4. Verify the shield bonding at every connector. The braided shield must be clamped to the D-sub backshell, not just touching the connector body.
5. Check the PC Adapter USB cable. A damaged USB cable or one that exceeds 5 m can cause the adapter to enumerate, light the green LED, but drop frames under load — appearing to STARTER as "no node found". Replace the cable and retest.

Common Engineering Pitfalls

• Wrong PG/PC interface handle: On a laptop that has both Drive ES STARTER and TIA Portal installed, the default PG/PC interface for S7ONLINE may be set to a TIA-bound Ethernet handle. Each tool installs its own handle, and the STARTER "Set PG/PC Interface" dialog must be re-opened after every TIA Portal upgrade.
• Service port X125 confusion: CU310-2 DP devices expose an Ethernet service port on X125 that supports Startdrive directly. CU310-DP (first-generation) does not have X125. Trying to reach the drive over Ethernet on a CU310-DP will never work.
• Multiple GSD files installed: Installing multiple SINAMICS GSD files in the hardware catalog can cause STARTER to bind to the wrong device description, which then refuses the online handshake. Clean the catalog and re-import the GSD for the S120 firmware build installed on the drive.
• RDY LED is not green: If the drive is in commissioning mode (first start, p0009 = 1 or p0344 = 1), it accepts parameter writes via BOP20 but may not publish the PROFIBUS interface until first commissioning is complete. Run the BOP20 quick commissioning routine to bring the drive to RUN, then retry the PG link.

Verification Procedure

Use this checklist to confirm the online link is fully functional before leaving the site.

1. From STARTER: Target device → Online → Accessible nodes returns the CU310 with its p0918 address. Time to detect: < 5 s at 1.5 Mbps.
2. From STARTER: Online → Go online succeeds and the project switches to the online view within 10 s.
3. From STARTER: read p0918 and verify it matches the value displayed on the BOP20.
4. From STARTER: read r2050[0] and r2050[1] and confirm they reflect the controller's PZD (control word and setpoint).
5. From STARTER: upload the parameter set (Target device → Load to PG) and archive it to the project folder. This proves read/write access and produces the as-commissioned record.
6. From BOP20: read r2053[0] (status word) and confirm the drive reports 0x0E37 (Ready, No fault, Operation enabled) once the controller has been placed in RUN and the drive has been given OFF1 = 1.
7. Document the PROFIBUS address, baud rate, telegram selection (p0922), and the PG/PC interface handle in the project cover page. This is required for any subsequent fault analysis.

FAQ